As image forming apparatuses such as a copying machine, a printer, etc., various image forming apparatuses using an electrophotographic scheme have been devised, and have become well known in the art. An image forming process includes a process such that an electrostatic latent image is formed on a surface of a photoconductive drum, which is an image-bearing body; the electrostatic latent image on the photoconductive drum is developed by a toner, which is a developer, etc., to produce a visualized image; the developed image is transferred to a recording sheet with a transferring apparatus to bear the image thereon; and the toner image on the recording sheet is fixed with a fixing apparatus which uses pressure, heat, etc.
In this fixing apparatus, a pressurizing member and a fixing member that include opposing rollers or belts or a combination thereof abut against each other to form a nip portion; a recording sheet is put into the nip portion, and heat and pressure are applied to fix the toner image onto the recording sheet.
An example of the fixing apparatus includes a known technique which uses, as a fixing member, a fixing belt which is stretched by multiple roller members (for example, see Patent Document 1). Such an apparatus using the fixing belt includes a fixing belt (an endless belt) 204 as a fixing member, multiple roller members 202, 203 which stretch and support the fixing belt 204, a heater 201 which is installed inside the roller member 202 of the multiple roller members 202, 203, a pressurizing roller (a pressurizing member) 205, etc. (see FIG. 19). The heater heats the fixing belt 204 via the roller members 202, 203. Then, a toner image on a recording medium which is carried to a nip portion formed between the fixing belt 204 and the pressurizing roller 205 is fixed onto the recording medium with heat and pressure received at the nip portion (Belt fixing scheme).
Moreover, for a fixing apparatus for use in the above-described image forming apparatus, there is a fixing apparatus which includes a fixed member which slidingly contacts an inner face of a fixing member which is a rotor.
For example, Patent document 2 discloses a film-heating fixing apparatus, wherein a heat-resistant film (a fixing film) 213 is sandwiched between a ceramic heater 211 as a heating body and a pressurizing roller 212 as a pressurizing member to form a fixing nip portion N, and a recording material with an unfixed toner image to be fixed being formed and carried thereon is introduced between the pressurizing roller 212 and the film 213 of the fixing nip portion N and is caused to be sandwiched and carried with the film 213 therebetween, so that, in the nip portion N, heat of the ceramic heater 211 is provided to the recording material via the film 213 and pressurizing of the fixing nip portion N fixes the unfixed toner image onto a face of the recording material by heat and pressure (see FIG. 20). This type of film-heating fixing apparatus allows configuring an on-demand type apparatus using a low heat capacity material as a film and a ceramic heater and also allows powering on the ceramic heater as a heat source to be heated to a predetermined fixing temperature only when executing image formation of an image forming apparatus, so that there are advantages that a waiting time is short from when the power of the image forming apparatus is turned on to when image forming can be executed (Quick start), and power consumed at standby is significantly small (power conservation), etc.
Moreover, in Patent documents 3 and 4 is proposed a pressurizing-belt image fixing apparatus, including a rotatable heating fixing roller, a surface of which rotatable heating fixing roll elastically deforms; an endless belt (a pressurizing belt) which allows travelling while being in contact to the heating fixing roller and a belt nip which is non-rotatably arranged inside the endless belt to abut the endless belt against the heating fixing roll and through which a recording sheet is caused to pass between the endless belt and the heating fixing roller; and a pressurizing pad which elastically deforms the surface of the heating fixing roller. According to this fixing scheme, a lower pressurizing member is arranged to be a belt and a contact area between a sheet and a roller is widened, making it possible to drastically improve heat conduction efficiency, suppress energy consumption, and realize a reduced size.
However, with the above-described fixing apparatus of Patent document 1, which is suitable for an increased speed of the apparatus relative to an apparatus using a fixing roller, there are limitations on reducing warm-up time (time required to reach a temperature at which printing is possible) and first-print time (from the time at which a request for printing is received to the time at which sheet discharging is completed via print preparation and printing operations).
On the other hand, the fixing apparatus of Patent document 2 makes it possible to shorten the warm-up time and the first print time due to a lowered heat capacity and to realize an apparatus of a reduced size. However, the fixing apparatus of Patent document 2 has problems with durability and with belt temperature stability. In other words, phenomena occur such as abrasion which is due to sliding of an inner face of a belt; a ceramic heater which is insufficient as a heat source; operating for a long time causing a face on which continuous friction is repeated to become rough, so that frictional resistance increases and travelling of the belt becomes unstable, or driving torque of the fixing apparatus increasing, so that, as a result, slipping of a transfer sheet for forming an image occurs; or stress on a drive gear increases, causing a failure such that the gear is damaged (Problem 1).
Moreover, with the film-heating fixing apparatus, there is a problem that, as the belt is locally heated at the nip portion, the belt temperature becomes lowest when the rotating belt returns to a nip inlet, so that, especially when high-speed rotating is conducted, fixing failure is likely to occur (Problem 2).
On the other hand, Patent document 3 discloses measures of improving the problem of slidability between a fixed member and an inner face of a belt using a polytetra-fluoroethylene (PTFE) impregnated glass fiber sheet (PTFE impregnated glass cloth) as a low friction sheet-shaped sliding material on a surface layer of a pressure pad. However, with such a pressurizing-belt fixing apparatus (Patent documents 3 and 4), there is a problem that heat capacity of the fixing roller is large and temperature rise is slow, so that time required for warming up is long (Problem 3).
For the problems 1 to 3 as described above, Patent document 5 discloses a fixing apparatus, wherein generally pipe-shaped opposing members (a metal heat conductor, a heating member, a supporting member) arranged on the inner peripheral side of an endless fixing belt and a ceramic heater which is arranged on the inner peripheral side of the opposing members and which heats the opposing members are provided, making it possible to heat the whole fixing belt, shorten warm-up time and first-print time, and overcome a shortage of heat capacity at the time of high-speed rotation.
However, with the fixing apparatus of Patent document 5, the nip portion which is formed by pressing, onto the fixing belt side, a pressurizing roller which is a pressurizing member is supported by a metal heat conductor, a nip width and pressure in the nip portion becomes unstable.
Thus, in the Patent document 6 is proposed the feature of providing, in order to maintain the state, shape, position, etc., of a pipe-shaped supporting member and a nip portion with a pressurizing roller and a fixing belt, nip forming members (an abutting member, a fixed member) and a reinforcing member such that they correspond to a part at which the nip portion is formed.